Recently an optical communication network in which high-speed and large-capacity data communication can be conducted is increasingly expanded. It is expected that the optical communication network is expanded to a commercial-off-the-shelf device from now. An electric-input-and-output optical data transmission cable (optical cable) which can be used like a current electric cable is demanded for the purpose of high-speed and large-capacity data transfer, a countermeasure against noise, and data transmission between boards in a device. Preferably a film optical waveguide is used as the optical cable in consideration of flexibility.
The optical waveguide is formed by a core and a clad, and an optical signal incident to the core propagates through the optical waveguide while total reflection is repeatedly generated at a boundary between the core and the clad. The core has a large refractive index. The clad having a small refractive index is provided around the core while contacting the core. The film optical waveguide has the flexibility because the core and the clad are made of flexible polymer materials.
In the case where the flexible film optical waveguide is used as the optical cable, it is necessary that the film optical waveguide be aligned with a photoelectric conversion element (light emission and acceptance element) to establish optical coupling. The light emission and acceptance element converts an electric signal into the optical signal to supply the optical signal, and receives the optical signal to converts the optical signal into the electric signal. In the light emission and acceptance element, a light emitting element is used on a light input side, and a light acceptance element is used on a light output side. High accuracy is required in the alignment, because the alignment has an influence on optical coupling efficiency.
In the case where the optical waveguide is used as the optical cable, there is adopted a method in which an insertion hole is made in a package and the optical waveguide is directly inserted into the insertion hole to fixed the optical waveguide to the package. For example, Patent Document 1 discloses an example of the method.
Patent Documents 2 and 3 disclose a method for fixing an optical waveguide in the case where a highly-flexible optical waveguide is used as the optical cable. Specifically, the optical waveguide is directly fixed to the light emission and acceptance element with an adhesive member such as a bonding agent.
Further, FIGS. 15(a) and 15(b) show a configuration example of an optical transmission module in which the film optical waveguide and the light emission and acceptance element are optically coupled to each other. The configuration shown in FIGS. 15(a) and 15(b) differs from the examples disclosed in Patent Documents 1 to 3.
In an end portion on an light incident side or a light outgoing side, an optical module 200 shown in FIGS. 15(a) and 15(b) includes an optical waveguide 201, a light emission and acceptance element 202, and a package 203. In the neighborhood of the end portion of the optical waveguide 201, the optical waveguide 201 is rigidly bonded to the package 203 with a bonding layer 204. A relative positional relationship between the end portion of the optical waveguide 201 and the light emission and acceptance element 202 is in a fixed state. At this point, in the light emission and acceptance element 202, the light-emitting element such as a laser diode is used on the light incident side to the optical waveguide 201, and the light acceptance element such as a photodiode is used on the light outgoing side from the optical waveguide 201.
The package 203 has a step such that a surface on which the light emission and acceptance element 202 is mounted is different from a surface (bonding surface) to which the optical waveguide 201 is fixed. An end face of the optical waveguide 201 is not perpendicular to an optical axis (center axis along a lengthwise direction of the core portion), but the end face is obliquely cut out to form an optical path changing mirror. Therefore, the signal light propagating through the core portion of the optical waveguide 201 is reflected by the optical path changing mirror, a traveling direction of the signal light is changed, and the signal light is emitted toward the light emission and acceptance element 202.
Patent Document 1: Japanese Patent Publication Laid-Open No. 6-82660 (published date of Mar. 25, 1994)
Patent Document 2: Japanese Patent Publication Laid-Open No. 2003-302544 (published date of Oct. 24, 2003)
Patent Document 3: Japanese Patent Publication Laid-Open No. 2004-21042 (published date of Jan. 22, 2004)
However, in the conventional configuration shown in FIGS. 15(a) and 15(b), depending on a type of usage of the optical module 200, reliability is lowered in rigidly bonding the optical waveguide 201 to the package 203. The reason why the reliability is lowered will be described below.
As described above, the highly-flexible optical waveguide 201 has the high flexibility because the core and the clad are made of flexible polymer materials. It is expected that the flexibility is put to use the optical waveguide 201 in data transmission in a movable portion such as a hinge coupling portion of a mobile device.
However, in the optical waveguide 201 used in the movable portion, a deformation is naturally generated in association with the movement. For example, when the optical module 200 is mounted on the hinge coupling portion of the mobile device, a tensile force is generated in an optical axis direction of the optical waveguide 201 by stretching the hinge portion.
When the tensile force is generated in the optical axis direction of the optical waveguide 201, a large amount of deformation is generated in the highly-flexible optical waveguide 201 while the deformation is hardly generated in the highly-rigid package 203. Therefore, a risk of peel-off or a breakage of the optical waveguide is generated in a bonding surface between the optical waveguide 201 and the package 203.
One or more embodiments of the present invention provides a highly-reliable optical module in which highly-flexible optical waveguide is stably joined to the package.